The present invention relates to the general field of optical devices. More specifically the invention relates to making optical measurements with reduced effects of amplified spontaneous emission.
There are a variety of test systems that attempt to accurately characterize the transmission and reflection properties of optical devices. Such optical devices include fiber-based filters, multiplexers, and other such optical devices. In order to meet the demands of today""s wavelength division multiplexed systems, it is important to make these measurements with high accuracy.
When using a laser to measure transmission through or reflection from a device, it is desired to measure the transmitted or reflected power only at a single wavelength, often referred to as the lasing wavelength. This laser power typically is measured using a photodetector. The ideal laser would only emit light at a single wavelength at any given instant. However, the presence of a broadband background of light at other wavelengths emitted by practical lasers, known as amplified spontaneous emission (ASE), in the light output beam from a laser at a given lasing wavelength adversely effects the ability to characterize the wavelength dependence of optical devices, degrading the accuracy of the measurement. More particularly, the amplified spontaneous emission limits the ability of a photodetector to specifically detect the lasing wavelength when making measurements since other wavelengths are detected by photodetectors. For example, measuring the high loss of filters with high contrast between stopbands and passbands can be difficult since the unattenuated ASE power transmitted through the passbands can be larger than the power of the lasing wavelength transmitted through the stopband. Referring to FIG. 6, a known wavelength transmission characteristic 600 for a notch filter is set forth. The filter is in the passband region at lasing wavelength 610, lasing wavelength 620, and lasing wavelength 640. At lasing wavelength 630, the filter is in the stopband region. The filter has a known loss of L1660 in the stopband. However, due to the effects of ASE in making optical measurements in the prior art, the loss at lasing wavelength 630 is measured to be only L2650.
In the past, several approaches have been used to reduce the effects of ASE. One approach is to design a better laser cavity which produces a laser output with lower amplified spontaneous emission. This approach is difficult, more expensive, and results in a lowered optical power output.
Another approach in the past is to use a tunable bandpass filter which passes only the lasing wavelength and blocks the unwanted ASE wavelengths from the detector. In order for such a filter to be effective, the filter should be capable of successfully tracking the lasing wavelength, and should have the desired filter shape to eliminate a substantial amount of the amplified spontaneous emission. Problems with this approach in the past include the expense and difficulty of both building the filter and controlling the filter. In addition, performance of the filter may be limited, as the filter may not adequately block the amplified spontaneous emissions, and use of the filter may cause a power ripple in the system.
As a result of the limitations in the prior art, there has been a need for a different, less expensive, and higher performance means to measure the power of a single wavelength of light in the presence of amplified spontaneous emission. The present invention meets this need.
The present invention provides a solution to the needs described above through an apparatus and method to minimize the effects of amplified spontaneous emission in optical measurements.
An embodiment of the invention presents an apparatus and method for minimizing the effects of noise when measuring a response of a device under test to laser light. Time-varying interference modulations are imparted to a laser light to produce a time-varying interference modulated test laser light and a time-varying interference modulated reference laser light. The time-varying interference modulated test laser light is propagated to the device under test, causing the device under test to propagate responsive time-varying interference modulated test laser light. The responsive time-varying interference modulated test laser light is converted to a responsive amplitude modulated electrical test signal and high pass filtered. The high pass filtered amplitude modulated electrical test signal is then processed to produce a time-varying electrical signal that represents the responsive time-varying interference modulated test laser light propagated by the device under test in response to the time varying interference modulated test laser light.
An embodiment of the invention presents an apparatus and method for minimizing the effects of noise when measuring a response of a device under test to laser light. Laser light provided from a laser source is scanned through a range of lasing light wavelengths so as to produce source laser light with a time-varying lasing light wavelength. This time-varying lasing light wavelength is provided to an interferometer with a first arm with a first optical path length and a second arm with a second optical path length. There is a path length difference between the first optical path length and the second optical path length. The source laser light is split into a first laser light portion and a second laser light portion. The first laser light portion travels down the first arm and the second laser light portion travel down the second arm and there is a difference in propagation time between the first laser light portion and the second laser light portion due to the path length difference. The first laser light portion and the second laser light portion are interferometrically combined, resulting in a time-varying interference modulated test laser light and a time-varying interference modulated reference laser light. The time-varying interference modulated test laser light is propagated to a device under test so as to cause the device under test to propagate responsive time-varying interference modulated test laser light. The responsive time-varying interference modulated test laser light is converted to an responsive amplitude modulated electrical test signal, which is high pass filtered. The high pass filtered signal is then processed so as to produce a time-varying electrical signal that represents the responsive time-varying interference modulated test laser light propagated by the device under test in response to the time varying interference modulated test laser light.
An embodiment of the invention presents another apparatus and method for minimizing the effects of noise when measuring a response of a device under test to laser light. Laser light at a prescribed wavelength is provided from a laser source. A time varying interferometer splits the laser light into a first laser light portion and a second laser light portion. The first laser light portion and second laser light portion travel different optical paths and the path length difference is time varying. The first laser light portion and the second laser light portion are interferometrically combined, producing a time-varying interference modulated test laser light and a time-varying interference modulated reference laser light. The time-varying interference modulated test laser light is propagated to a device under test so as to cause the device under test to propagate responsive time-varying interference modulated test laser light. The responsive time-varying interference modulated test laser light is converted to an responsive amplitude modulated electrical test signal, which is high pass filtered. The high pass filtered signal is then processed so as to produce a time-varying electrical signal that represents the responsive time-varying interference modulated test laser light propagated by the device under test in response to the time varying interference modulated test laser light.
An embodiment of the invention presents another apparatus and method for minimizing the effects of noise when measuring a response of a device under test to laser light. Laser light at a lasing wavelength is provided from a laser source. The phase or wavelength of the lasing light is modulated. This time-varying lasing light is provided to an interferometer with a first arm with a first optical path length and a second arm with a second optical path length. There is a path length difference between the first optical path length and the second optical path length. The source laser light is split into a first laser light portion and a second laser light portion. The first laser light portion travels down the first arm and the second laser light portion travels down the second arm and there is a difference in propagation time between the first laser light portion and the second laser light portion due to the path length difference. The first laser light portion and the second laser light portion are interferometrically combined, producing a time-varying interference modulated test laser light and a time-varying interference modulated reference laser light. The time-varying interference modulated test laser light is propagated to a device under test so as to cause the device under test to propagate responsive time-varying interference modulated test laser light. The responsive time-varying interference modulated test laser light is converted to an responsive amplitude modulated electrical test signal, which is high pass filtered. The high pass filtered signal is then processed so as to produce a time-varying electrical signal that represents the responsive time-varying interference modulated test laser light propagated by the device under test in response to the time varying interference modulated test laser light.
In an embodiment of the invention, a method for removing the effects of amplified spontaneous emission upon the response of a device under test to a lasing light is presented. The method comprises providing a high frequency time-varying lasing light, and propagating the high frequency time varying lasing light to a device under test, which results in a responsive high frequency time-varying lasing light at the output of the device under test. This responsive signal is converted to a high frequency electrical signal, and then high pass filtered to reduce the effects of amplified spontaneous emission.